Plating methods for covering the surface of a metal product with a thin film of metal are commonly used to improve the appearance or surface protection of metal products.
Sporting equipment that is plated in this manner to reduce susceptibility to damage is also known, as disclosed in JP 2001-000600 A. In the disclosed sporting equipment, a golf club head having a stainless steel member is plated with a film on at least the stainless steel member, wherein the film has a Vickers hardness of 500 or greater. This plating is formed in two or more layers composed of a bright nickel plating layer and a hard plating layer.
According to testing by the inventors, the bright nickel plating layer or the hard plating layer has a Vickers hardness that is 500 or greater, and is close to 600.
However, it has become apparent that the durable lifetime of the sporting equipment becomes shorter than expected when the Vickers hardness is 500 to 600, as a result of the combined effects of multiple factors such as increased frequency of use of the sporting equipment. This problem can most likely be overcome by increasing the Vickers hardness to 700 or greater, and preferably to 800.
Heat treatment is known as a method for increasing the hardness of a plating layer, as disclosed in JP H11-302856 A. FIGS. 6A and 6B hereof show the heat treatment method disclosed in JP H11-302856 A.
In FIG. 6A, a substrate is hardened in step (hereinafter abbreviated as ST) 101. The hardened substrate is then plated in ST102. The plated substrate is then tempered in ST103.
As shown in FIG. 6B, the unmodified substrate having a hardness of 200 (micro Vickers hardness) is hardened to a hardness of 800 or greater by heat treatment (tempering) at 400° C. Specifically, when the tempering temperature in ST103 is 400° C., precipitation occurs in the plating film, and precipitation hardening progresses. As a result, the hardness of the plating film increases dramatically.
Hardening and tempering of the substrate are accompanied by expansion during heating, and contraction during cooling of the substrate. The substrate sometimes does not completely return to the original shape after this contraction. This dimensional disruption makes it necessary to include a correction step. Since large external forces are applied to the plating film during this correction, the plating film is sometimes destroyed.
A method exists for heating only the plating film and causing precipitation hardening in order to strengthen the plating film. Even in this case, the substrate is expected to flex or distort due to heating. There is therefore a need for a hardening/strengthening method that does not rely on heat treatment or heating.